Optical disc reproducing apparatus

ABSTRACT

An objective lens is raised from below upwards with a DVD laser diode turned on and levels of detection signals A, B, C, and D from a four segment photodetector are acquired and stored in a RAM. A ratio (A+D)/(B+C) in a direction of the X-axis is then calculated. When the ratio exceeds a predetermined value, a lens shift correction amount is calculated for the calculated ratio based on a correction table. The objective lens is then shifted in the direction of the X-axis in accordance with the calculated lens shift correction amount. These processes are executed before a series of processes required for achieving a focus-on state.

This application is based on Japanese Patent Application No. 2006-308871filed on Nov. 15, 2006, the contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disc reproducing apparatusfor reproducing information recorded on an optical disc, andparticularly relates to processing for correcting deviation of lightreceived by a photodetector caused by lens shifting at the time ofsearch operation by an optical pick-up.

2. Description of Related Arts

Conventionally, with an optical disc reproducing apparatus, a trackingservo for an optical pick-up is carried out in accordance with atracking error signal. More specifically, the tracking servo for theoptical pick-up is carried out in accordance with a track cross signalwhere pulse-width changes according to an amount of shifting (amount ofdeviation toward an inner periphery-side or outer periphery-side of atrack on the optical disc) of an objective lens provided in the opticalpick-up relative to a sled on which the optical pick-up is mounted.

For example, when the objective lens is shifted to the inner peripheryside of a track on the optical disc, as a result of a tracking servobased on a track cross signal at this time, in order to return theobjective lens to a normal position with respect to the track, trackingcontrol is performed. In this tracking control, a tracking drive voltageis applied to a tracking actuator so that the objective lens is moved ina direction towards the outer periphery side with the objective lensthen adopting a normal position with respect to the track.

However, with conventional optical disc reproducing apparatus, theprocessing described in the following is carried out by the systemcontroller until the focus servo goes on. Here, a description is givenwhere a DVD is loaded into the optical disc reproducing apparatus.

The system controller then turns a DVD laser diode on. A sum signal(A+B+C+D) and a focus error signal (A+C)−(B+D) are then acquired from afour segment photodetector. Next, the DVD laser diode is turned off, anda CD laser diode is turned on. Then, as described above, the sum signal(A+B+C+D) and the focus error signal (A+C)−(B+D) are acquired.

After this, when the system controller determines that the inserted discis a DVD as a result of the level of the sum signal (A+B+C+D) obtainedwith the DVD laser diode turned on being larger than the level of thesum signal (A+B+C+D) obtained with the CD laser diode turned on, thelevels of detection signals A, B, C, and D from the four segmentphotodetector with the DVD laser diode turned on are individuallyadjusted so as to all be the same level. Next, the system controllercarries out S-shaped signal balance adjustment of the acquired focuserror signal and focus pulling-in setting for pulling in the focus servowith the DVD laser diode turned on. The focus servo is then turned on.

With optical disc reproducing apparatus of the related art, afterturning the focus servo on and achieving a focus-on state using theprocessing described above, the tracking servo is turned on, and a spotof laser light from the optical pick-up is tracked on to a track on theoptical disc. However, in processing for up to the focus servo beingturned on as described above, when detracking (a state where thetrack-on is achieved at a location deviating from the center of thetrack) occurs, laser light is irradiated on a location deviating fromthe center of the track even when a focus-on state is achieved with thestate remaining as is. As a result, jitter of the reproduced signalworsens and the quality of reproduction from the optical disc falls.

In the technology of the related art disclosed in JP-A-H09-326123, atthe time of start-up or re-starting of the apparatus, when a convergentlens is moved away after coming close to the disc, or when a convergentlens comes close to the disc after being moved away, an amplitude of anS-shaped signal appearing on the focus error signal each time aconvergent point of the optical beam passes through each informationsurface is measured. The focus system gain is then switched over so asto give a predetermined amplitude, and an optimum pulling-in level isset. However, in the technology of the related art, countermeasures forthe case where detracking occurs are not disclosed for the processing upto the focus servo going on, and the problem described above thereforecannot be solved.

In the technology of the related art disclosed in JP-A-H04-252435,(A+D)−(B+C) is calculated and main information containing a trackingerror component and an offset error component accompanying shift of anobjective lens is created. Next, (A+B)−(C+D) is calculated, a transformcoefficient is applied to the result so as to give a transformation toan offset error component. Correction information is then created, andtracking control is carried out. However, in this technology of therelated art, countermeasures for the case where detracking occurs arenot disclosed for the processing up to the focus servo going on. Theproblems described above therefore cannot be solved. Further, whenoptical discs that are subject to reproduction are, for example,DVD-ROMs and DVD-RWs, reflectance of the DVD-ROMs is greater thanreflectance of the DVD-RWs. This means that the result for (A+D)−(B+C)of the DVD-ROMs is large compared to that of the DVD-RWs. When a numberof types of optical disc are handled, the results of calculating(A+D)−(B+C) become different and become therefore ambiguous. This meansthat the precision of tracking control is effectively lowered.

In the related technology disclosed in JP-A-2005-203062, an amplitude ofa phase difference tracking error signal is measured in the vicinity ofan end section of an inner periphery-side and in the vicinity of an endsection of an outer periphery-side within a recording region of anoptical disc and minimum values are extracted. Both of the extractedminimum values are then compared, and the signal of the smalleramplitude is selected. An amplification rate for a phase differencetracking error signal is then decided in such a manner that it is alwayspossible to detect the phase difference tracking error signal based onthis smaller signal and a value set in advance. However, in thistechnology of the related art also, countermeasures for the case wheredetracking occurs are not disclosed for the processing up to the focusservo going on and the problem described above therefore cannot besolved.

SUMMARY OF THE INVENTION

In order to solve the problems described above, it is the object of thepresent invention to provide an optical disc reproducing apparatus thatis capable of alleviating detracking at the time of an operation ofsearching an optical disc that is a target of reproduction using anoptical pick-up by detecting an amount of deviation of light received ina radial direction of a disc by a photodetector of an optical pick-up,and making correction by moving the objective lens of the opticalpick-up in a direction that cancels the amount of deviation.

To achieve this object, optical disc reproduction apparatus of thepresent invention is comprised of: a photodetector divided into fourlight-receiving regions of a first quadrant light-receiving region, asecond quadrant light-receiving region, a third quadrant light-receivingregion, and a fourth quadrant light-receiving region by an X-axis and aY-axis in X-Y coordinates when a radial direction of an optical disc istaken to be the X-axis and a track direction of the optical disc istaken to be the Y-axis; an optical pick-up for irradiating laser lightfrom a DVD (Digital Versatile Disc) laser diode or a CD (Compact Disc)laser diode onto the optical disc via an objective lens, receiving lightreflected from the optical disc at the photodetector, and outputtingfirst, second, third, and fourth detection signals of the first, second,third, and fourth quadrant light-receiving regions, respectively; atracking servo section for shifting the objective lens in the radialdirection of the optical disc; a focus servo section for shifting theobjective lens in a vertical direction with respect to a recordingsurface of the optical disc; and a moving section for moving the opticalpick-up in the radial direction of the optical disc. The optical discreproduction apparatus further comprises a system controller, the systemcontroller comprising: a DVD signal level acquiring and storing sectionfor moving the objective lens upwards with the CD laser diode turned offand the DVD laser diode turned on, and acquiring levels of the first,second, third, and fourth detection signals from the photodetector forstorage in a memory; a DVD ratio calculating section for calculating aratio (A+D)/(B+C) in a direction of the X-axis when the levels of thefirst, second, third, and fourth detection signals stored in the memoryare taken to be A, B, C, and D, respectively; a DVD lens shiftcorrection amount calculating section for calculating a lens shiftcorrection amount for the calculated ratio based on a correction tableindicating a relationship of the ratio and the lens shift correctionamount when the calculated ratio exceeds a predetermined value; and aDVD lens shift section for controlling the tracking servo section inaccordance with the lens shift correction amount that is calculated soas to shift the objective lens in the direction of the X-axis when thecalculated ratio exceeds the predetermined value. The system controllerexecutes processing provided by said sections included therein andreflects results in a succeeding series of processes required forachieving a focus-on state.

Moreover, optical disc reproduction apparatus of the present inventionis comprised of: a photodetector divided into four light-receivingregions of a first quadrant light-receiving region, a second quadrantlight-receiving region, a third quadrant light-receiving region, and afourth quadrant light-receiving region by an X-axis and a Y-axis in X-Ycoordinates when a radial direction of an optical disc is taken to bethe X-axis and a track direction of the optical disc is taken to be theY-axis; an optical pick-up for irradiating laser light from a DVD laserdiode or a CD laser diode onto the optical disc via an objective lens,receiving light reflected from the optical disc at the photodetector,and outputting first, second, third, and fourth detection signals of thefirst, second, third, and fourth quadrant light-receiving regions,respectively; a tracking servo section for shifting the objective lensin the radial direction of the optical disc; a focus servo section forshifting the objective lens in a vertical direction with respect to arecording surface of the optical disc; and a moving section for movingthe optical pick-up in the radial direction of the optical disc. Theoptical disc reproduction apparatus further comprises a systemcontroller, the system controller comprising: a CD signal levelacquiring and storing section for moving the objective lens upwards withthe DVD laser diode turned off and the CD laser diode turned on, andacquiring levels of the first, second, third, and fourth detectionsignals from the photodetector for storage in the memory; a CD ratiocalculating section for calculating a ratio (A+D)/(B+C) in a directionof the X-axis when the levels of the first, second, third, and fourthdetection signals stored in the memory are taken to be A, B, C, and D,respectively; a CD lens shift correction amount calculating section forcalculating a lens shift correction amount for a calculated ratio basedon a correction table indicating a relationship of the ratio and thelens shift correction amount when the calculated ratio exceeds apredetermined value; and a CD lens shift section for controlling thetracking servo section in accordance with the lens shift correctionamount that is calculated so as to shift the objective lens in thedirection of the X-axis when the calculated ratio exceeds thepredetermined value. The system controller executes processing providedby said sections included therein and reflects results in a succeedingseries of processes required for achieving a focus-on state.

Still further, optical disc reproduction apparatus of the presentinvention is comprised of: a photodetector divided into fourlight-receiving regions of a first quadrant light-receiving region, asecond quadrant light-receiving region, a third quadrant light-receivingregion, and a fourth quadrant light-receiving region by an X-axis and aY-axis in X-Y coordinates when a radial direction of an optical disc istaken to be the X-axis and a track direction of the optical disc istaken to be the Y-axis; an optical pick-up for irradiating laser lightfrom a DVD laser diode or a CD laser diode onto the optical disc via anobjective lens, receiving light reflected from the optical disc at thephotodetector, and outputting first, second, third, and fourth detectionsignals of the first, second, third, and fourth quadrant light-receivingregions, respectively; a tracking servo section for shifting theobjective lens in the radial direction of the optical disc; a focusservo section for shifting the objective lens in a vertical directionwith respect to a recording surface of the optical disc; and a movingsection for moving the optical pick-up in the radial direction of theoptical disc. The optical disc reproduction apparatus further comprisesa system controller, the system controller comprising: a DVD signallevel acquiring and storing section for moving the objective lensupwards with the CD laser diode turned off and the DVD laser diodeturned on, and acquiring levels of the first, second, third, and fourthdetection signals for DVD use from the photodetector for storage in thememory; a DVD ratio calculating section for calculating a ratio(A+D)/(B+C) for DVD use in a direction of the X-axis when the levels ofthe first, second, third, and fourth detection signals for DVD usestored in the memory are taken to be A, B, C, and D, respectively; a DVDlens shift correction amount calculating section for calculating a lensshift correction amount for the calculated ratio for DVD use based on acorrection table indicating a relationship of the calculated ratio forDVD use and the lens shift correction amount for DVD use when thecalculated ratio exceeds a predetermined value; a DVD lens shift sectionfor controlling the tracking servo section in accordance with the lensshift correction amount for DVD use that is calculated so as to shiftthe objective lens in the direction of the X-axis when the calculatedratio for DVD use exceeds the predetermined value; a CD signal levelacquiring and storing section for moving the objective lens upwards withthe DVD laser diode turned off and the CD laser diode turned on, andacquiring the levels of the first, second, third and fourth detectionsignals from the photodetector for storage in the memory; a CD ratiocalculating section for calculating a ratio (A+D)/(B+C) for CD use inthe direction of the X-axis when the levels of the first, second, third,and fourth detection signals for CD use stored in the memory are takento be A, B, C, and D, respectively; a CD lens shift correction amountcalculating section for calculating a lens shift correction amount forthe calculated ratio for CD use based on a correction table indicating arelationship of the calculated ratio for CD use and a lens shiftcorrection amount for CD use when the calculated ratio exceeds apredetermined value; and a CD lens shift section for controlling thetracking servo section in accordance with the lens shift correctionamount for CD use that is calculated so as to shift the objective lensin the direction of the X-axis when the calculated ratio for CD useexceeds the predetermined value. The system controller executesprocessing provided by said sections included therein and reflectsresults in a succeeding series of processes required for achieving afocus-on state.

According to the above configuration, the system controller adds asearch operation that is an operation of acquiring the detection signalsA, B, C, and D of the photodetector by moving the objective lens upwardswith the DVD laser diode turned on and calculates the ratio in thedirection of the X-axis after acquiring the levels of the detectionsignals A, B, C, and D. The lens shift correction amount due to theadded search operation is then adopted only when this ratio is largerthan a predetermined value. Therefore, the amount of deviation of lightreceived at a light-receiving region of the photodetector in a directionof the X-axis is detected at the time of the search operation. It isthen possible to alleviate detracking by moving the objective lens in adirection canceling this deviation and then making correction. DVDreproduction quality is therefore improved as a result.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block view showing a configuration of an optical discreproducing apparatus of first, second, and third embodiments of thepresent invention;

FIG. 2 is a block view illustrating an operation of an objective lens, atracking actuator, and a focus actuator incorporated in theconfiguration of the optical pick-up shown in FIG. 1;

FIG. 3 is a view showing a light-receiving region of a four segmentphotodetector of the first, second, and third embodiments;

FIG. 4 is a view showing an example of a correction table illustratingthe relationship between a ratio α and a lens shift correction amount ofthe first embodiment;

FIG. 5 is a timing chart illustrating individual steps leading up to thefocus-on state being achieved after a process for a search operation isadditionally executed in the beginning while a DVD laser diode of theoptical pick-up is kept on in the first embodiment;

FIG. 6 is a flowchart illustrating an operation leading up to thefocus-on state being achieved after the process for a search operationis additionally executed in the beginning while the DVD laser diode ofthe optical pick-up is kept on in the first embodiment;

FIG. 7 is a timing chart illustrating individual steps leading up to thefocus-on state being achieved after the process for a search operationis additionally executed in the beginning while the CD laser diode ofthe optical pick-up is kept on in the second embodiment; and

FIG. 8 is a timing chart illustrating individual steps leading up to thefocus-on state being achieved after the process for the search operationis additionally executed in the beginning while the DVD laser diode orthe CD laser diode of the optical pick-up is kept on in the thirdembodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the attached drawings. FIG. 1 is a block view showinga configuration for an optical disc reproducing apparatus of first,second, and third embodiments of the present invention. In FIG. 1, thisoptical disc reproducing apparatus is provided with: an optical pick-up3 for emitting laser light to reproduce information recorded on anoptical disc 1 and receiving light reflected from the optical disc 1; aspindle motor 2 for rotating the optical disc 1; a spindle drive circuit19 for driving this spindle motor 2; a tracking drive circuit 17 fordriving a tracking actuator 31 (refer to FIG. 2) for carrying out atracking servo for the optical pick-up 3; a focus drive circuit 16 fordriving a focus actuator 32 (refer to FIG. 2) for carrying out the focusservo for the optical pick-up 3; a sled 4 for moving the optical pick-up3 along in a radial direction of the optical disc; and a sled drivecircuit 18 for driving this sled 4.

This optical disc reproducing apparatus is also provided with thefollowing: an RF amplifier 5 for receiving a reproduction signal(read-out signal) from the optical pick-up 3 during reproduction of theoptical disc 1, the RF amplifier 5 then generating an RF signal fromthis reproduction signal and amplifying the generated RF signal; adata/synchronization signal separating circuit 6 provided with a PLL(phase-locked loop) 8 containing a VCO (voltage controlled oscillator),for receiving the RF signal from the RF amplifier 5 and separating theRF signal into data and a synchronization signal; a data decode errorcorrection circuit 9 for receiving data separated by thedata/synchronization signal separating circuit 6, carrying out errorchecking by performing decoding, and carrying out error correction so asto output correct data when an error exists in the data; and an AVdecode circuit 10 for receiving the correct data from the data decodeerror correction circuit 9, decoding the data, and outputting a videosignal and an audio signal.

Further, the disc reproducing apparatus is provided with the following:a mirror detection circuit 7 for detecting mirror signals indicatingmirror surfaces having no tracks on the optical disc 1 and contained inthe RF signal from the RF amplifier 5 and for counting the mirrorsurfaces; a tracking error detection circuit 11 for detecting a trackingerror signal contained in the reproduction signal from the opticalpick-up 3; a focus error detection circuit 12 for detecting a focuserror signal contained in the reproduction signal from the opticalpick-up 3; and a track cross detection circuit 13 for detecting trackcrossing based on the tracking error signal from the tracking errordetection circuit 11 and outputting a track pulse.

Further, the optical disc reproducing apparatus is provided with asystem controller 20 for controlling each of the above configurationelements in accordance with a CPU 15 carrying out processing for theapparatus as a whole. A flash ROM 21 storing programs and data etc.required for the control by the system controller 20 and a RAM 14 fortemporarily storing data required for the operations and processing bythe CPU 15 are connected to the CPU 15.

The system controller 20 carries out control such as: control for thefocus servo of the optical pick-up 3 based on the focus error signalcontained in the reproduction signal from the optical pick-up 3; controlfor the tracking servo of the optical pick-up 3 based on the trackingerror signal contained in the reproduction signal from the opticalpick-up 3; control for driving the sled 4 via the sled drive circuit 18so as to move the optical pick-up 3 in a radial direction of the opticaldisc 1; and control for rotating the spindle motor 2 via the spindledrive circuit 19.

The system controller 20 of the first embodiment is provided with thefollowing: a DVD signal level acquiring and storing section for movingan objective lens 33 upwards with a DVD laser diode in an on-state so asto acquire levels A, B, C, and D of first, second, third, and fourthdetection signals from the photodetector 37 for storage in the RAM 14; aDVD ratio calculating section for calculating a ratio α=(A+D)/(B+C) in adirection of the X-axis from these signal levels; a DVD lens shiftcorrection amount calculating section for calculating the lens shiftcorrection amount for the calculated ratio α based on a correction tableindicating a relationship of the ratio and the lens shift correctionamount when the ratio α exceeds a predetermined value; a DVD lens shiftsection for controlling a tracking servo section in accordance with thelens shift correction amount so as to shift the objective lens 33 in thedirection of the X-axis when the ratio α exceeds the predeterminedvalue; a DVD sum signal/focus error signal acquiring section foracquiring the sum signal for the detection signals A, B, C, D and thefocus error signal with a DVD laser diode 361 turned on after reflectingthe results of processing by the DVD signal level acquiring and storingsection, the DVD ratio calculating section, the DVD lens shiftcorrection amount calculating section, and the DVD lens shift section; aCD sum signal/focus error signal acquiring section for acquiring the sumsignal for the detection signals A, B, C, and D and the focus errorsignal with the DVD laser diode 361 turned off and a CD laser diode 362turned on; a DVD signal level adjusting section for individuallyadjusting the signal levels A, B, C, and D so that all of the levels arethe same level by adjusting the gain of the RF amplifier 5 with the DVDlaser diode turned on when it is determined that an inserted disc is aDVD because the level of the sum signal obtained by the DVD sumsignal/focus error signal acquiring section is larger than the level ofthe sum signal obtained by the CD sum signal/focus error signalacquiring section, after reflecting the results of processing by the DVDsignal level acquiring and storing section, the DVD ratio calculatingsection, the DVD lens shift correction amount calculating section, andthe DVD lens shift section; and a DVD S-shaped signal balanceadjustment/focus pulling-in setting section for performing S-shapedsignal balance adjustment of the acquired focus error signal and settingfocus pulling-in in order to pull in the focus servo with the DVD laserdiode turned on after reflecting the results of processing by the DVDsignal level acquiring and storing section, the DVD ratio calculatingsection, the DVD lens shift correction amount calculating section, andthe DVD lens shift section.

FIG. 2 is a block view illustrating an operation of the objective lens,the tracking actuator, and the focus actuator incorporated in aconfiguration of an optical pick-up shown in FIG. 1. In FIG. 2, numeral36 represents a light source having the DVD laser diode 361 and the CDlaser diode 362 respectively irradiating laser light. Laser lightirradiated from the light source 36 passes through a hologram element 35and is reflected by a mirror 34. The laser light is then focused by theobjective lens 33 and is irradiated in the form of a spot onto arecording surface of the optical disc 1. After being focused again bythe objective lens 33 and reflected by the mirror 34, reflected lightreflected by the recording surface of the optical disc 1 is divided intoa plurality of light beams while passing through the hologram element 35and becomes incident to the four segment photodetector 37.

A +/−1 order light component diffracted by a track groove on therecording surface of the optical disc 1 is included in light reflectedby the optical disc 1 and focused by the objective lens 33. Positionsetting is then carried out in such a way that first and second regionsformed by dividing into two to the left and right with respect to atrack groove direction at the surface of the hologram element 35 arepositioned so that a light signal modulated by a +1 order light or −1order light component passes through. Reflected light passing throughthe first and second regions at the hologram element 35 is then dividedinto different directions, received by the four segment photodetector37, and photoelectrically converted. The tracking error detectioncircuit 11 then selects predetermined signals from individual signalsoutputted by the four segment photodetector 37. A tracking error signalis then generated from the difference between the selected signals. Thefocus error detection circuit 12 then selects predetermined signals fromthe individual signals outputted by the four segment photodetector 37. Afocus error signal is then generated from the difference between theselected signals.

The lens holder 38 on which the objective lens 33 is mounted issupported in a freely moveable manner by the focus actuator 32 so as tomove in a vertical direction with respect to the recording surface ofthe optical disc 1. Therefore, the lens holder 38 is driven by the focusactuator 32. A focus drive voltage passing through the focus drivecircuit 16 based on the focus error signal outputted by the focus errordetection circuit 12 is then supplied to the focus actuator 32, andfocus servo control is carried out.

Further, the lens holder 38 is supported in a freely moveable manner bythe tracking actuator 31 so as to move in a radial direction (trackingdirection) of the optical disc 1 with respect to the sled 4. The lensholder 38 is therefore driven by the tracking actuator 31. A trackingdrive voltage passing through the tracking drive circuit 17 based on thetracking error signal outputted by the tracking error detection circuit11 is supplied to the tracking actuator 31 and tracking servo control iscarried out.

At this time, the lens holder 38, i.e. the objective lens 33 is shiftedin the tracking direction in accompaniment with eccentricity of theoptical disc 1. The rotational frequency (eccentric frequency) of theoptical disc 1 is high at, for example, a number of Hz to a number oftens of Hz, while a frequency range of a moving operation for the sled 4is low at, for example, 1 Hz. This means that only the objective lens 33follows the track groove of the optical disc 1 during the tracking servoand the objective lens 33 is therefore shifted with respect to the sled4 due to eccentricity of the track groove.

FIG. 3 is a view showing a light-receiving region of the four segmentphotodetector 37 of the first, second, and third embodiments. In FIG. 3,the X-axis indicates the radial direction of the optical disc, and theY-axis indicates the track direction of the optical disc. In XYcoordinates, a light-receiving region a is arranged at a first quadrant,a light-receiving region b is arranged at a second quadrant, alight-receiving region c is arranged at a third quadrant, and alight-receiving region d is arranged at a fourth quadrant. Further,symbol A denotes a detection signal detected by the light-receivingregion a, symbol B denotes a detection signal detected by thelight-receiving region b, symbol C denotes a detection signal detectedby the light-receiving region c, and symbol D denotes a detection signaldetected by the light-receiving region d. The tracking error signal istherefore (A+D)−(B+C), and the focus error signal obtained by anastigmatism method is (A+C)−(B+D). In particular, in this embodiment, aratio α=(A+D)/(B+C) of the (A+D) signal level and the (B+C) signal levelis calculated. Hereafter, the tracking error signal, the focus errorsignal, and the ratio α denote the calculated results using the aboveoperation unless otherwise specified.

FIG. 4 is a view showing an example of a correction table illustratingthe relationship between the ratio α and the lens shift correctionamount of the first embodiment. In the correction table shown in FIG. 4,the ratio α denotes the aforementioned ratio (A+D)/(B+C) in dB(decibels). A limit for the lens shift correction amount is set to ±110μm. If the ratio α is −3.0 dB, then the lens shift correction amount is+110 μm. When the ratio α is greater than −3.0 dB and less than −1.5 dB,then the lens shift correction amount is +55 μm. When the ratio α isgreater than −1.5 dB and less than +1.5 dB, the lens shift correctionamount is 0 μm and lens shift correction is not performed. Namely, whenthe ratio α is within ±1.5 dB, the lens shift correction amount is 0 μmand lens shift correction is not performed. When the ratio α is greaterthan +1.5 dB and less than +3 dB, then the lens shift correction amountis −55 μm. If the ratio α is +3.0 dB, then the lens shift correctionamount is −110 μm. The extent of movement of the objective lens 33 isset taking into consideration the range of possible movement of thelens.

FIG. 5 is a timing chart illustrating individual steps leading up to afocus-on state being achieved after a process for a search operation (anoperation where an objective lens is moved upwards and levels ofdetection signals A, B, C, and D are obtained from the four segmentphotodetector by the system controller) is additionally executed in thebeginning while the DVD laser diode of the optical pick-up is kept on inthe first embodiment.

In FIG. 5, a line L indicates a voltage outputted by the focus drivecircuit 16 and supplied to the focus actuator 32. A direction upwardsfor this voltage denoted by the line L corresponds to a direction (adirection approaching the recording surface of the optical disc 1) ofmovement of the objective lens 33 upwards. The line L thereforeindicates change in level of a voltage applied by the focus drivecircuit 16 to the focus actuator 32.

First, in a period T1, the detection signals A, B, C, and D are acquiredfrom the four segment photodetector 37 with the CD laser diode 362 offand the DVD laser diode 361 on. A ratio α of the (A+D) signal level andthe (B+C) signal level that is the ratio in the direction of the X-axisis then calculated. When the ratio α exceeds a predetermined value,control is carried out to shift the objective lens 33 in the directionof the X-axis (a direction for canceling deviation of the receivedlight).

In a period T2, a sum signal (A+B+C+D) that is a reproduction signal (RFsignal) and a focus error signal (A+C)−(B+D) are acquired with the DVDlaser diode 361 turned on. The level of the sum signal at this time isused in discerning that the disc type of the inserted disc is a DVD.Further, the sum signal and the focus error signal at this time are usedin amplitude adjustment of the respective signals.

In a period T3, the sum signal and the focus error signal are acquiredwith the DVD laser diode 361 turned off and the CD laser diode 362turned on. The level of the sum signal at this time is used indiscerning that the disc type of the inserted disc is a CD. Further, thesum signal and the focus error signal at this time are used in amplitudeadjustment of the respective signals.

In a period T4, individual amplitude adjustment of the detection signalsA, B, C, and D for the DVD is carried out. In a period T5, S-shapedsignal balance adjustment of the focus error signal and setting of focuspulling-in are carried out for the DVD. In a period T6, a focus-on stateis achieved at a time t6.

The above operation is now described in further detail. In the periodT1, the DVD signal level acquiring and storing section of the systemcontroller 20 raises the objective lens 33 upwards with the CD laserdiode 362 turned off and the DVD laser diode 361 turned on so as toacquire the levels of the detection signals A, B, C, and D from the foursegment photodetector 37 for storing in the RAM 14. The DVD ratiocalculating section of the system controller 20 then refers to thelevels of the detection signals A, B, C, and D stored in the RAM 14 andcalculates the ratio α in the direction of the X-axis. When thecalculated ratio α exceeds a predetermined value, the DVD lens shiftcorrection amount calculating section calculates the lens shiftcorrection amount for the calculated ratio α based on the correctiontable indicating the relationship of the ratio and the lens shiftcorrection amount (refer to FIG. 4). Further, when the calculated ratioα exceeds the predetermined value, the DVD lens shift section controlsthe tracking drive circuit 17 in accordance with the calculated lensshift correction amount so as to shift the objective lens 33 in thedirection of the X-axis (in a direction canceling deviation of thereceived light).

In the period T2, the results of processing by the DVD signal levelacquiring and storing section, the DVD ratio calculating section, theDVD lens shift correction amount calculating section, and the DVD lensshift section are reflected. The DVD sum signal/focus error signalacquiring section of the system controller 20 acquires the sum signaland the focus error signal from the detection signals A, B, C, and Dwith the DVD laser diode 361 turned on.

In the period T3, the CD sum signal/focus error signal acquiring sectionof the system controller 20 acquires the sum signal and the focus errorsignal from the detection signals A, B, C, and D with the DVD laserdiode 361 turned off and the CD laser diode 362 turned on.

In the period T4, the results of processing by the DVD signal levelacquiring and storing section, the DVD ratio calculating section, theDVD lens shift correction amount calculating section, and the DVD lensshift section are reflected. When it is determined that an inserted discis a DVD because the level of the sum signal acquired by the DVD sumsignal/focus error signal acquiring section is larger than the level ofthe sum signal acquired by the CD sum signal/focus error signalacquiring section, the DVD signal level adjusting section of the systemcontroller 20 individually adjusts the levels of the detection signalsA, B, C, and D to all be the same level by adjusting the gain of the RFamplifier 5 with the CD laser diode 362 turned off and the DVD laserdiode 361 turned on. Further, in the period T4, the system controller 20makes the gain of the focus error detection circuit 12 high, raises thelevel of the focus error signal up to the target level, and carries outa search operation.

In the period T5, the results of processing (the lens shift correctionamount) by the DVD signal level acquiring and storing section, the DVDratio calculating section, the DVD lens shift correction amountcalculating section, and the DVD lens shift section are reflected. TheDVD S-shaped signal balance adjustment/focus pulling-in setting sectionof the system controller 20 carries out S-shaped signal balanceadjustment and setting of focus pulling-in in order to pull in the focusservo for the focus error signal that is acquired, with the DVD laserdiode 361 turned on. In the period T6, the focusing is achieved at thetime t6 using the focus servo performed by the focus error detectioncircuit 12, the focus drive circuit 16, and the focus actuator 32.

In this focus pulling-in setting, the sum signal and the focus errorsignal are measured. A first condition is then established when theamplitude of the focus error signal exceeds half of a predeterminedvalue of the focus error signal. After this first condition isestablished, a second condition is then established when the amplitudeof the sum signal exceeds half of a predetermined value of the sumsignal. When the objective lens 33 is then shifted in an upwarddirection with this situation maintained, and when a search operation isperformed on the optical disc 1, the focusing becomes just-in-focus, anda third condition is established. The focus pulling-in setting cantherefore be said to be setting in order to pull in the focus servo byestablishing the first, second, and third conditions.

FIG. 6 is a flowchart illustrating, in the first embodiment, how asearch operation (an operation where levels of the detection signals A,B, C, and D from the four segment photodetector are acquired by thesystem controller while moving the objective lens in an upwarddirection) with the DVD laser diode of the optical pick-up turned on isinitially added one time, and illustrates each step thereafter up to thefocus-on state being achieved. A description is now given with referenceto this flowchart and FIG. 1 to FIG. 5 of an operation for initiallyadding a search operation where the DVD laser diode of the opticalpick-up is turned on one time, and of the operation thereafter up to thefocus-on state being achieved.

First, the DVD signal level acquiring and storing section of the systemcontroller 20 turns the DVD laser diode 361 on (step S1) and controlsthe focus drive circuit 16 so that the focus actuator 32 is driven andthe objective lens 33 is temporarily lowered (step S2). The DVD signallevel acquiring and storing section of the system controller 20 thencontrols the focus drive circuit 16 in the period T1, drives the focusactuator 32, raises the objective lens 33, obtains the levels of thedetection signals A, B, C, and D from the four segment photodetector 37respectively (step S3), and stores them in the RAM 14 (step S4).

In the period T1, the DVD ratio calculating section of the systemcontroller 20 calculates the ratio (A+D)/(B+C) in the direction of theX-axis and converts this ratio to the ratio α denoted in decibels (stepS5). In the period T1, when the ratio α exceeds a predetermined value,the DVD lens shift correction amount calculating section of the systemcontroller 20 calculates the lens shift correction amount for the ratioα based on the correction table indicating the relationship of the ratioand the lens shift correction amount (refer to FIG. 4) (step S6). Whenthe ratio α exceeds the predetermined value, in the period T1, the DVDlens shift section controls the tracking drive circuit 17 in accordancewith the lens shift correction amount that is calculated and drives thetracking actuator 31. The objective lens 33 is then shifted in thedirection of the X-axis that is a direction that cancels deviation oflight received by the four segment photodetector 37 (step S7).

Next, in the period T2, the DVD sum signal/focus error signal acquiringsection of the system controller 20 reflects the results of theprocessing (the lens shift correction amount) by the DVD signal levelacquiring and storing section, the DVD ratio calculating section, theDVD lens shift correction amount calculating section, and the DVD lensshift section. The sum signal and the focus error signal are thenacquired with the DVD laser diode 361 turned on (step S8).

After this, in the period T3, the CD sum signal/focus error signalacquiring section of the system controller 20 returns the output of thetracking drive circuit 17 to zero in order not to reflect the amount oflens correction. The sum signal and the focus error signal are thenacquired with the DVD laser diode 361 turned off and the CD laser diode362 turned on (step S9).

Next, in the period T4, the DVD signal level adjusting section of thesystem controller 20 reflects the results of processing (the lens shiftcorrection amount) by the DVD signal level acquiring and storingsection, the DVD ratio calculating section, the DVD lens shiftcorrection amount calculating section, and the DVD lens shift section.When it is determined that the inserted disc is a DVD (step S10) becausethe level of the sum signal acquired by the DVD sum signal/focus errorsignal acquiring section is larger than the level of the sum signalacquired by the CD sum signal/focus error signal acquiring section, thelevels of the detection signals A, B, C, and D are individually adjustedto all be the same level by adjusting the gain of the RF amplifier 5with the CD laser diode 362 turned off and the DVD laser diode 361turned on (step S11).

Next, in the period T5, the DVD S-shaped signal balance adjustment/focuspulling-in setting section of the system controller 20 reflects theresults of processing (the lens shift correction amount) by the DVDsignal level acquiring and storing section, the DVD ratio calculatingsection, the DVD lens shift correction amount calculating section, andthe DVD lens shift section. S-shaped signal balance adjustment of theacquired focus error signal and focus pulling-in setting in order topull in the focus servo is then carried out with the CD laser diode 362turned off and the DVD laser diode 361 turned on (step S12). In theperiod T6, the system controller 20 turns the focusing on at time t6(step S13) using a focusing servo performed by the focus error detectioncircuit 12, the focus drive circuit 16, and the focus actuator 32.

According to the first embodiment, the system controller 20 carries outan additional search operation that is an operation for acquiring thedetection signals A, B, C, and D of the photodetector 37 by moving theobjective lens 33 upwards with the DVD laser diode 361 turned on andacquires the levels of the detection signals A, B, C, and D. After this,the ratio α in the direction of the X-axis is calculated, and the lensshift correction amount resulting from the added search operation isadopted only when the ratio α is larger than the predetermined value. Itis therefore possible to alleviate detracking at the time of a searchoperation by detecting an amount of deviation of light received by thelight-receiving regions a, b, c, and d of the photodetector 37 in thedirection of the X-axis, moving the objective lens 33 in a directioncanceling this deviation, and making correction. As a result,reproduction quality for the DVD is increased.

FIG. 7 is a timing chart illustrating how a search operation (anoperation where levels of the detection signals A, B, C, and D from thefour segment photodetector are acquired by the system controller whilemoving the objective lens in an upward direction) with the DVD laserdiode of the optical amplifier turned on is initially added one time,and illustrates each step thereafter up to the focus-on state beingachieved in the first embodiment.

The system controller 20 of a second embodiment is provided with thefollowing: a CD signal level acquiring and storing section for movingthe objective lens 33 upwards with the DVD laser diode 361 turned offand the CD laser diode 362 turned on, and acquiring the levels of thefirst, second, third, and fourth detection signals A, B, C, and D fromthe four segment photodetector 37 for storage in the RAM 14; a CD ratiocalculating section for calculating a ratio α=(A+D)/(B+C) in thedirection of the X-axis from these signal levels; a CD lens shiftcorrection amount calculating section for calculating a lens shiftcorrection amount for the calculated ratio α based on a correction tableindicating a relationship of the ratio and the lens shift correctionamount when the ratio α exceeds a predetermined value; a CD lens shiftsection for controlling the tracking servo section in accordance withthe lens shift correction amount that is calculated so as to shift theobjective lens 33 in the direction of the X-axis when the ratio αexceeds the predetermined value; a DVD sum signal/focus error signalacquiring section for acquiring the sum signal for the detection signalsA, B C, and D and the focus error signal with the CD laser diode 362turned off and the DVD laser diode 361 turned on; a CD sum signal/focuserror signal acquiring section for acquiring the sum signal for thedetection signals A, B, C, and D and the focus error signal with the DVDlaser diode 361 turned off and the CD laser diode 362 turned on so as toreflect the processing results by the CD signal level acquiring andstoring section, the CD ratio calculating section, the CD lens shiftcorrection amount calculating section, and the CD lens shift section; aCD signal level adjusting section for individually adjusting the signallevels A, B, C, and D so that all of the levels are the same level byadjusting the gain of the RF amplifier 5 with the DVD laser diode 361turned off and the CD laser diode 362 turned on when it is determinedthat an inserted disc is a CD because the level of the sum signalobtained by the CD total signal/focus error signal acquiring section islarger than the level of the sum signal obtained by the DVD sumsignal/focus error signal acquiring section, after reflecting results ofprocessing by the CD signal level acquiring and storing section, the CDratio calculating section, the CD lens shift correction amountcalculating section, and the CD lens shift section; and a CD S-shapedsignal balance adjustment/focus pulling-in setting section forperforming S-shaped signal balance adjustment of the acquired focuserror signal and setting focus pulling-in in order to pull in the focusservo with the DVD laser diode 361 turned off and the CD laser diode 362turned on after reflecting the results of processing by the CD signallevel acquiring and storing section, the CD ratio calculating section,the CD lens shift correction amount calculating section, and the CD lensshift section.

In FIG. 7, a line L indicates a voltage outputted by the focus drivecircuit 16 and supplied to the focus actuator 32. An upward direction ofthis voltage denoted by the line L corresponds to a direction ofmovement of the objective lens 33 upwards. The line L thereforeindicates change in level of a voltage applied by the focus drivecircuit 16 to the focus actuator 32.

First, in a period T1, the detection signals A, B, C, and D are acquiredfrom the four segment photodetector 37 with the DVD laser diode 361turned off and the CD laser diode 362 turned on. A ratio α of the (A+D)signal level and the (B+C) signal level that is the ratio in thedirection of the X-axis is then calculated. When the ratio α exceeds apredetermined value, control is carried out to shift the objective lens33 in the direction of the X-axis.

In a period T2, a sum signal (A+B+C+D) that is a reproduction signal (RFsignal) and a focus error signal (A+C)−(B+D) are acquired with the CDlaser diode 362 off and the DVD laser diode 361 on. The level of the sumsignal at this time is used in discerning that the disc type of theinserted disc is a DVD. Further, the sum signal and the focus errorsignal at this time are used in amplitude adjustment of the respectivesignals.

In a period T3, the sum signal and the focus error signal are acquiredwith the DVD laser diode 361 off and the CD laser diode 362 on. Thelevel of the sum signal at this time is used in discerning that the disctype of the inserted disc is a CD. Further, the sum signal and the focuserror signal at this time are used in amplitude adjustment of therespective signals.

In a period T4, individual amplitude adjustment of detection signals A,B, C, and D for the CD is carried out. In a period T5, S-shaped signalbalance adjustment of the focus error signal and setting of focuspulling-in is carried out for the CD. In a period T6, a focus-on stateis achieved at a time t6.

The above operation is now described in further detail. In the periodT1, the CD signal level acquiring and storing section of the systemcontroller 20 raises the objective lens 33 upwards with the DVD laserdiode 361 turned off and the CD laser diode 362 turned on so as toacquire the levels of the detection signals A, B, C, and D from the foursegment photodetector 37 for storing in the RAM 14. The CD ratiocalculating section of the system controller 20 then refers to thelevels of the detection signals A, B, C, and D stored in the RAM 14 andcalculates the ratio α in the direction of the X-axis. When thecalculated ratio α exceeds a predetermined value, the CD lens shiftcorrection amount calculating section calculates the lens shiftcorrection amount for the calculated ratio α based on the correctiontable indicating the relationship of the ratio and the lens shiftcorrection amount. Further, when the calculated ratio α exceeds thepredetermined value, the CD lens shift section controls the trackingdrive circuit 17 in accordance with the calculated lens shift correctionamount so as to shift the objective lens 33 in the direction of theX-axis.

In the period T2, as the results of processing (lens shift correctionamount) by the CD signal level acquiring and storing section, the CDratio calculating section, and the CD lens shift correction amountcalculating section are not reflected, and the DVD sum signal/focuserror signal acquiring section of the system controller 20 acquires thesum signal and the focus error signal from the detection signals A, B,C, and D with the CD laser diode 362 turned off and the DVD laser diode361 turned on.

In the period T3, the CD sum signal/focus error signal acquiring sectionof the system controller 20 reflects the results of processing (the lensshift correction amount) by the CD signal level acquiring and storingsection, the CD ratio calculating section, the CD lens shift correctionamount calculating section, and the CD lens shift section. The sumsignal and the focus error signal are then acquired with the DVD laserdiode 361 turned off and the CD laser diode 362 turned on.

In the period T4, the results of processing (the lens shift correctionamount) by the CD signal level acquiring and storing section, the CDratio calculating section, the CD lens shift correction amountcalculating section, and the CD lens shift section are reflected. Whenit is determined that an inserted disc is a CD because the level of thesum signal acquired by the CD sum signal/focus error signal acquiringsection is larger than the level of the sum signal acquired by the DVDsum signal/focus error signal acquiring section, the CD signal leveladjustment section of the system controller 20 therefore individuallyadjusts the levels of the detection signals A, B, C, and D to all be thesame level by adjusting the gain of the RF amplifier 5 with the DVDlaser diode 361 turned off and the CD laser diode 362 turned on.Further, in the period T4, the system controller 20 makes the gain ofthe focus error detection circuit 12 high, raises the level of the focuserror signal up to the target level, and carries out a search operation.

In the period T5, the results of processing (the lens shift correctionamount) by the CD signal level acquiring and storing section, the CDratio calculating section, the CD lens shift correction amountcalculating section, and the CD lens shift section are reflected. The CDS-shaped signal balance adjustment/focus pulling-in setting section ofthe system controller 20 carries out S-shaped signal balance adjustmentand setting of focus pulling-in in order to pull in the focus servo forthe focus error signal that is acquired, with the DVD laser diode 361turned off and the CD laser diode 362 turned on. In the period T6, thefocusing is achieved at the time t6 using the focus servo performed bythe focus error detection circuit 12, the focus drive circuit 16, andthe focus actuator 32.

According to the second embodiment, the system controller 20 carries outan additional search operation that is an operation for acquiring thedetection signals A, B, C, and D of the photodetector 37 by moving theobjective lens 33 upwards with the DVD laser diode 361 turned off andthe CD laser diode 362 turned on, and acquires the levels of thedetection signals A, B, C, and D. After this, the ratio α in thedirection of the X-axis is calculated, and the lens shift correctionamount resulting from the added search operation is adopted only whenthe ratio α is larger than the predetermined value. It is thereforepossible to alleviate detracking at the time of a search operation bydetecting an amount of deviation of light received by thelight-receiving regions a, b, c, and d of the photodetector 37 in thedirection of the X-axis, moving the objective lens 33 in a directioncanceling this deviation, and making correction. As a result,reproduction quality for the CD is increased.

FIG. 8 is a timing chart illustrating how a search operation (anoperation where levels of the detection signals A, B, C, and D from thefour segment photodetector are acquired by the system controller whilemoving the objective lens in an upward direction) with the DVD laserdiode or the CD laser diode of the optical amplifier in an on state isinitially added one time, and illustrates each step thereafter up to thefocus-on state being achieved in a third embodiment.

The system controller 20 of the third embodiment is provided with thefollowing: a DVD signal level acquiring and storing section for raisingthe objective lens 33 upwards with the CD laser diode 362 turned off andthe DVD laser diode 361 turned on and acquiring the levels of the first,second, third, and fourth detection signals A, B, C, and D from the foursegment photodetector 37 for storage in the RAM 14; a DVD ratiocalculating section for calculating a ratio α=(A+D)/(B+C) in a directionof the X-axis from these signal levels; a DVD lens shift correctionamount calculating section for calculating a lens shift correctionamount for the calculated ratio α based on a correction table indicatinga relationship of the ratio and the lens shift correction amount whenthe ratio α exceeds a predetermined value; a DVD lens shift section forcontrolling the tracking servo section in accordance with the lens shiftcorrection amount that is calculated so as to shift the objective lens33 in the X-direction when the ratio α exceeds the predetermined value;a CD signal level acquiring and storing section for moving the objectivelens 33 upwards with the DVD laser diode 361 turned off and the CD laserdiode 362 turned on and acquiring the levels of the first, second,third, and fourth detection signals A, B, C, and D, respectively, fromthe four segment photodetector 37 for storage in the RAM 14; a CD ratiocalculating section for calculating a ratio α=(A+D)/(B+C) in a directionof the X-axis from these signal levels; a CD lens shift correctionamount calculating section for calculating a lens shift correctionamount for the calculated ratio α based on a correction table indicatinga relationship of the ratio α and the lens shift correction amount whenthe ratio α exceeds a predetermined value; and a CD lens shift sectionfor controlling the tracking servo section in accordance with the lensshift correction amount that is calculated so as to shift the objectivelens 33 in the direction of the X-axis when the ratio α that iscalculated exceeds the predetermined value.

The system controller 20 is further provided with the following: a DVDsum signal/focus error signal acquiring section for acquiring the sumsignal for the detection signals A, B C, and D and the focus errorsignal with the CD laser diode 362 turned off and the DVD laser diode361 turned on; a CD sum signal/focus error signal acquiring section foracquiring the sum signal for the detection signals A, B, C, and D andthe focus error signal with the DVD laser diode 361 turned off and theCD laser diode 362 turned on after reflecting the processing results bythe CD signal level acquiring and storing section, the CD ratiocalculating section, the CD lens shift correction amount calculatingsection, and the CD lens shift section; a DVD signal level adjustingsection for individually adjusting the levels of the signals A, B, C,and D to all be the same level by adjusting the gain of the RF amplifier5 with the CD laser diode 362 turned off and the DVD laser diode 361turned on after reflecting the results of processing by the DVD signallevel acquiring and storing section, the DVD ratio calculating section,the DVD lens shift correction amount calculating section, and the DVDlens shift section when the inserted disc is determined to be a DVDbecause the level of the sum signal acquired by the DVD sum signal/focuserror signal acquiring section is larger than the level of the sumsignal acquired by the CD sum signal/focus error signal acquiringsection; a CD signal level adjusting section for individually adjustingthe levels of the signals A, B, C, and D to all be the same level byadjusting the gain of the RF amplifier 5 with the DVD laser diode 361turned off and the CD laser diode 362 turned on after reflecting theresults of processing by the CD signal level acquiring and storingsection, the CD ratio calculating section, the CD lens shift correctionamount calculating section, and the CD lens shift section when theinserted disc is determined to be a CD because the level of the sumsignal acquired by the CD sum signal/focus error signal acquiringsection is larger than the level of the sum signal acquired by the DVDsum signal/focus error signal acquiring section; a DVD S-shaped signalbalance adjustment/focus pulling-in setting section for performingS-shaped signal balance adjustment of the acquired focus error signaland setting focus pulling-in in order to pull in the focus servo withthe CD laser diode 362 turned off and the DVD laser diode 361 turned onafter reflecting the results of processing by the DVD signal levelacquiring and storing section, the DVD ratio calculating section, theDVD lens shift correction amount calculating section, and the DVD lensshift section; and a CD S-shaped signal balance adjustment/focuspulling-in setting section for performing S-shaped signal balanceadjustment of the acquired focus error signal and setting focuspulling-in in order to pull in the focus servo with the DVD laser diode361 turned off and the CD laser diode 362 turned on after reflecting theresults of processing by the CD signal level acquiring and storingsection, the CD ratio calculating section, the CD lens shift correctionamount calculating section, and the CD lens shift section.

In FIG. 8, a line L indicates a voltage outputted by the focus drivecircuit 16 and supplied to the focus actuator 32. An upward directionfor this voltage denoted by the line L corresponds to a direction ofmovement of the objective lens 33 upwards. The line L thereforeindicates change in level of a voltage applied by the focus drivecircuit 16 to the focus actuator 32.

First, in a period T1D, the detection signals A, B, C, and D areacquired from the four segment photodetector 37 with the CD laser diode362 turned off and the DVD laser diode 361 turned on. A ratio α of the(A+D) signal level and the (B+C) signal level that is the ratio in thedirection of the X-axis is then calculated. When the ratio α exceeds apredetermined value, control is carried out to shift the objective lens33 in the direction of the X-axis.

In a period T1C, the detection signals A, B, C, and D are acquired fromthe four segment photodetector 37 with the DVD laser diode 361 off andthe CD laser diode 362 on. A ratio α of the (A+D) signal level and the(B+C) signal level that is the ratio in the direction of the X-axis isthen calculated. When the ratio α exceeds a predetermined value, controlis carried out to shift the objective lens 33 in the direction of theX-axis.

In a period T2, a sum signal (A+B+C+D) that is a reproduction signal (RFsignal) and a focus error signal (A+C)−(B+D) are acquired with the CDlaser diode 362 off and the DVD laser diode 361 on. The level of the sumsignal at this time is used in discerning that the disc type of theinserted disc is a DVD. Further, the sum signal and the focus errorsignal at this time are used in amplitude adjustment of the respectivesignals.

In a period T3, the sum signal and the focus error signal are acquiredwith the DVD laser diode 361 off and the CD laser diode 362 on. Thelevel of the sum signal at this time is used in discerning that the disctype of the inserted disc is a CD. Further, the sum signal and the focuserror signal at this time are used in amplitude adjustment of therespective signals.

In a period T4D, individual amplitude adjustment of the detectionsignals A, B, C, and D for the DVD is carried out. A line L1 occurringin the period T4D indicates a focus drive voltage (voltage applied tothe focus actuator 32) for the DVD. In the period T4C, individualamplitude adjustment of the detection signals A, B, C, and D for the CDis carried out. A line L2 occurring in the period T4C indicates a focusdrive voltage for the CD.

In a period T5D, S-shaped signal balance adjustment of the focus errorsignal and setting of focus pulling-in is carried out for the DVD. In aperiod T5C, S-shaped signal balance adjustment of the focus error signaland setting of focus pulling-in is carried out for the CD. In a periodT6D, the focus-on state is achieved for the DVD. In a period T6C, thefocus-on state is achieved for the CD.

The above operation is now described in further detail. In the periodT1D, the DVD signal level acquiring and storing section of the systemcontroller 20 moves the objective lens 33 upwards with the CD laserdiode 362 turned off and the DVD laser diode 361 turned on so as toacquire the levels of the detection signals A, B, C, and D from the foursegment photodetector 37 for storing in the RAM 14. The DVD ratiocalculating section of the system controller 20 then refers to thelevels of the detection signals A, B, C, and D stored in the RAM 14 andcalculates the ratio α in the X-direction. When the calculated ratio αexceeds a predetermined value, the DVD lens shift correction amountcalculating section calculates a lens shift correction amount for thecalculated ratio α based on the correction table indicating therelationship of the ratio and the lens shift correction amount. Further,when the calculated ratio α exceeds the predetermined value, the DVDlens shift section controls the tracking drive circuit 17 in accordancewith the calculated lens shift correction amount so as to shift theobjective lens 33 in the direction of the X-axis.

In the period T1C, the CD signal level acquiring and storing section ofthe system controller 20 moves the objective lens 33 upwards with theDVD laser diode 361 turned off and the CD laser diode 362 turned on soas to acquire the levels of the detection signals A, B, C, and D fromthe four segment photodetector 37 for storing in the RAM 14. The CDratio calculating section of the system controller 20 then refers to thelevels of the detection signals A, B, C, and D stored in the RAM 14 andcalculates the ratio α in the X-direction. When the calculated ratio αexceeds a predetermined value, the CD lens shift correction amountcalculating section calculates a lens shift correction amount for thecalculated ratio α based on the correction table indicating therelationship of the ratio and the lens shift correction amount. Further,when the calculated ratio α exceeds the predetermined value, the CD lensshift section controls the tracking drive circuit 17 in accordance withthe calculated lens shift correction amount so as to shift the objectivelens 33 in the direction of the X-axis.

In the period T2, the results of processing (the lens shift correctionamount) by the DVD signal level acquiring and storing section, the DVDratio calculating section, the DVD lens shift correction amountcalculating section, and the DVD lens shift section are reflected. TheDVD sum signal/focus error signal acquiring section of the systemcontroller 20 acquires a sum signal and the focus error signal from thedetection signals A, B, C, and D with the CD laser diode 362 turned offand the DVD laser diode 361 turned on.

In the period T3, the CD sum signal/focus error signal acquiring sectionof the system controller 20 reflects the results of processing (the lensshift correction amount) by the CD signal level acquiring and storingsection, the CD ratio calculating section, the CD lens shift correctionamount calculating section, and the CD lens shift section. The sumsignal and the focus error signal are then acquired from the detectionsignals A, B, C, and D with the DVD laser diode 361 turned off and theCD laser diode 362 turned on.

In the period T4D, the results of processing (the lens shift correctionamount) by the DVD signal level acquiring and storing section, the DVDratio calculating section, the DVD lens shift correction amountcalculating section, and the DVD lens shift section are not reflected.When it is determined that an inserted disc is a DVD because the levelof the sum signal acquired by the DVD sum signal/focus error signalacquiring section is larger than the level of the sum signal acquired bythe CD sum signal/focus error signal acquiring section, the DVD signallevel adjusting section of the system controller 20 individually adjuststhe levels of the detection signals A, B, C, and D to all be the samelevel by adjusting the gain of the RF amplifier 5 with the CD laserdiode 362 turned off and the DVD laser diode 361 turned on. Further, inthe period T4D, the system controller 20 makes the gain of the focuserror detection circuit 12 high, raises the level of the focus errorsignal up to the target level, and carries out a search operation.

In the period T4C, the results of processing (the lens shift correctionamount) by the CD signal level acquiring and storing section, the CDratio calculating section, the CD lens shift correction amountcalculating section, and the CD lens shift section are reflected. Whenit is determined that an inserted disc is a CD because the level of thesum signal acquired by the CD sum signal/focus error signal acquiringsection is larger than the level of the sum signal acquired by the DVDsum signal/focus error signal acquiring section, the CD signal leveladjustment section of the system controller 20 individually adjusts thelevels of the detection signals A, B, C, and D to all be the same levelby adjusting the gain of the RF amplifier 5 with the DVD laser diode 361turned off and the CD laser diode 362 turned on. Further, in the periodT4C, the system controller 20 makes the gain of the focus errordetection circuit 12 high, raises the level of the focus error signal upto the target level, and carries out a search operation.

In the period T5D, the results of processing (the lens shift correctionamount) by the DVD signal level acquiring and storing section, the DVDratio calculating section, the DVD lens shift correction amountcalculating section, and the DVD lens shift section are reflected. TheDVD S-shaped signal balance adjustment/focus pulling-in setting sectionof the system controller 20 carries out S-shaped signal balanceadjustment and setting of focus pulling-in in order to pull in the focusservo for the focus error signal that is acquired, with the CD laserdiode 362 turned off and the DVD laser diode 361 turned on.

In the period T5C, the results of processing (the lens shift correctionamount) by the CD signal level acquiring and storing section, the CDratio calculating section, the CD lens shift correction amountcalculating section, and the CD lens shift section are reflected. The CDS-shaped signal balance adjustment/focus pulling-in setting section ofthe system controller 20 carries out S-shaped signal balance adjustmentand setting of focus pulling-in in order to pull in the focus servo forthe focus error signal that is acquired, with the DVD laser diode 361turned off and the CD laser diode 362 turned on. In a period T6D, thefocus-on state is achieved for the DVD. In a period T6C, the focus-onstate is achieved for the CD.

According to the third embodiment, the system controller 20 carries outan additional search operation that is an operation for acquiring thedetection signals A, B, C, and D of the photodetector 37 by moving theobjective lens 33 upwards with the CD laser diode 362 turned off and theDVD laser diode 361 turned on, and acquires the levels of the detectionsignals A, B, C, and D. After this, the ratio α in the direction of theX-axis is calculated, and the lens shift correction amount resultingfrom the added search operation is adopted only when the ratio α islarger than the predetermined value. It is therefore possible toalleviate detracking at the time of a search operation by detecting anamount of deviation of light received by the light-receiving regions a,b, c, and d of the photodetector 37 in the direction of the X-axis,moving the objective lens 33 in a direction canceling this deviation,and making correction. As a result, reproduction quality for the DVD isincreased.

Further, the system controller 20 carries out an additional searchoperation that is an operation for acquiring the detection signals A, B,C, and D of the photodetector 37 by moving the objective lens 33 upwardswith the DVD laser diode 361 turned off and the CD laser diode 362turned on, and acquires the levels of the detection signals A, B, C, andD. After this, the ratio α in the direction of the X-axis is calculated,and the lens shift correction amount resulting from the added searchoperation is adopted only when the ratio α is larger than thepredetermined value. It is therefore possible to alleviate detracking atthe time of a search operation by detecting an amount of deviation oflight received by the light-receiving regions a, b, c, and d of thephotodetector 37 in the direction of the X-axis, moving the objectivelens 33 in a direction canceling this deviation, and making correction.As a result, reproduction quality for the CD is increased.

1. Optical disc reproduction apparatus comprising: a photodetectordivided into four light-receiving regions of a first quadrantlight-receiving region, a second quadrant light-receiving region, athird quadrant light-receiving region, and a fourth quadrantlight-receiving region by an X-axis and a Y-axis in X-Y coordinates whena radial direction of an optical disc is taken to be the X-axis and atrack direction of the optical disc is taken to be the Y-axis; anoptical pick-up for irradiating laser light from a DVD (DigitalVersatile Disc) laser diode or a CD (Compact Disc) laser diode onto theoptical disc via an objective lens, receiving light reflected from theoptical disc at the photodetector, and outputting first, second, third,and fourth detection signals of the first, second, third, and fourthquadrant light-receiving regions, respectively; a tracking servo sectionfor shifting the objective lens in the radial direction of the opticaldisc; a focus servo section for shifting the objective lens in avertical direction with respect to a recording surface of the opticaldisc; and a moving section for moving the optical pick-up in the radialdirection of the optical disc, wherein the optical disc reproductionapparatus further comprises a system controller, the system controllercomprising: a DVD signal level acquiring and storing section for movingthe objective lens upwards with the CD laser diode turned off and theDVD laser diode turned on, and acquiring levels of the first, second,third, and fourth detection signals from the photodetector for storagein a memory; a DVD ratio calculating section for calculating a ratio(A+D)/(B+C) in a direction of the X-axis when the levels of the first,second, third, and fourth detection signals stored in the memory aretaken to be A, B, C, and D, respectively; a DVD lens shift correctionamount calculating section for calculating a lens shift correctionamount for the calculated ratio based on a correction table indicating arelationship of the ratio and the lens shift correction amount when thecalculated ratio exceeds a predetermined value; and a DVD lens shiftsection for controlling the tracking servo section in accordance withthe lens shift correction amount that is calculated so as to shift theobjective lens in the direction of the X-axis when the calculated ratioexceeds the predetermined value, wherein the system controller executesprocessing provided by said sections included therein and reflectsresults in a succeeding series of processes required for achieving afocus-on state.
 2. The optical disc reproduction apparatus of claim 1,wherein the system controller further comprises: a DVD sum signal/focuserror signal acquiring section for acquiring a DVD sum signal (A+B+C+D)for the first, second, third, and fourth detection signals and a DVDfocus error signal (A+C)−(B+D) with the CD laser diode turned off andthe DVD laser diode turned on, after reflecting results of processing bythe DVD signal level acquiring and storing section, the DVD ratiocalculating section, the DVD lens shift correction amount calculatingsection, and the DVD lens shift section; a CD sum signal/focus errorsignal acquiring section for acquiring a CD sum signal for the first,second, third, and fourth detection signals and a CD focus error signalwith the DVD laser diode turned off and the CD laser diode turned on; aDVD signal level adjusting section for individually adjusting the levelsof the signals A, B, C, and D with the CD laser diode turned off and theDVD laser diode turned on to all be the same level when it is determinedthat an inserted disc is a DVD because a level of the DVD sum signal islarger than a level of the CD sum signal, after reflecting the resultsof processing by the DVD signal level acquiring and storing section, theDVD ratio calculating section, the DVD lens shift correction amountcalculating section, and the DVD lens shift section; and a DVD S-shapedsignal balance adjustment/focus pulling-in setting section forperforming S-shaped signal balance adjustment of the DVD focus errorsignal and setting focus pulling-in in order to pull in a focus servowith the CD laser diode turned off and the DVD laser diode turned on,after reflecting the results of processing by the DVD signal levelacquiring and storing section, the DVD ratio calculating section, theDVD lens shift correction amount calculating section, and the DVD lensshift section.
 3. Optical disc reproduction apparatus comprising: aphotodetector divided into four light-receiving regions of a firstquadrant light-receiving region, a second quadrant light-receivingregion, a third quadrant light-receiving region, and a fourth quadrantlight-receiving region by an X-axis and a Y-axis in X-Y coordinates whena radial direction of an optical disc is taken to be the X-axis and atrack direction of the optical disc is taken to be the Y-axis; anoptical pick-up for irradiating laser light from a DVD laser diode or aCD laser diode onto the optical disc via an objective lens, receivinglight reflected from the optical disc at the photodetector, andoutputting first, second, third, and fourth detection signals of thefirst, second, third, and fourth quadrant light-receiving regions,respectively; a tracking servo section for shifting the objective lensin the radial direction of the optical disc; a focus servo section forshifting the objective lens in a vertical direction with respect to arecording surface of the optical disc; and a moving section for movingthe optical pick-up in the radial direction of the optical disc, whereinthe optical disc reproduction apparatus further comprises a systemcontroller, the system controller comprising: a CD signal levelacquiring and storing section for moving the objective lens upwards withthe DVD laser diode turned off and the CD laser diode turned on, andacquiring levels of the first, second, third, and fourth detectionsignals from the photodetector for storage in the memory; a CD ratiocalculating section for calculating a ratio (A+D)/(B+C) in a directionof the X-axis when the levels of the first, second, third, and fourthdetection signals stored in the memory are taken to be A, B, C, and D,respectively; a CD lens shift correction amount calculating section forcalculating a lens shift correction amount for a calculated ratio basedon a correction table indicating a relationship of the ratio and thelens shift correction amount when the calculated ratio exceeds apredetermined value; and a CD lens shift section for controlling thetracking servo section in accordance with the lens shift correctionamount that is calculated so as to shift the objective lens in thedirection of the X-axis when the calculated ratio exceeds thepredetermined value, wherein the system controller executes processingprovided by said sections included therein and reflects results in asucceeding series of processes required for achieving a focus-on state.4. The optical disc reproduction apparatus of claim 3, wherein thesystem controller further comprises: a DVD sum signal/focus error signalacquiring section for acquiring a DVD sum signal (A+B+C+D) for thefirst, second, third, and fourth detection signals and a DVD focus errorsignal (A+C)−(B+D) with the CD laser diode turned off and the DVD laserdiode turned on; a CD sum signal/focus error signal acquiring sectionfor acquiring a CD sum signal for the first, second, third, and fourthdetection signals and a CD focus error signal with the DVD laser diodeturned off and the CD laser diode turned on, after reflecting results ofprocessing by the CD signal level acquiring and storing section, the CDratio calculating section, the CD lens shift correction amountcalculating section, and the CD lens shift section; a CD signal leveladjusting section for individually adjusting the levels of the signalsA, B, C, and D with the DVD laser diode turned off and the CD laserdiode turned on to all be the same level when it is determined that aninserted disc is a CD because a level of the CD sum signal is largerthan a level of the DVD sum signal, after reflecting the results ofprocessing by the CD signal level acquiring and storing section, the CDratio calculating section, the CD lens shift correction amountcalculating section, and the CD lens shift section; and a CD S-shapedsignal balance adjustment/focus pulling-in setting section forperforming S-shaped signal balance adjustment of the CD focus errorsignal and setting focus pulling-in in order to pull in a focus servowith the DVD laser diode turned off and the CD laser diode turned on,after reflecting the results of processing by the CD signal levelacquiring and storing section, the CD ratio calculating section, the CDlens shift correction amount calculating section, and the CD lens shiftsection.
 5. Optical disc reproduction apparatus comprising: aphotodetector divided into four light-receiving regions of a firstquadrant light-receiving region, a second quadrant light-receivingregion, a third quadrant light-receiving region, and a fourth quadrantlight-receiving region by an X-axis and a Y-axis in X-Y coordinates whena radial direction of an optical disc is taken to be the X-axis and atrack direction of the optical disc is taken to be the Y-axis; anoptical pick-up for irradiating laser light from a DVD laser diode or aCD laser diode onto the optical disc via an objective lens, receivinglight reflected from the optical disc at the photodetector, andoutputting first, second, third, and fourth detection signals of thefirst, second, third, and fourth quadrant light-receiving regions,respectively; a tracking servo section for shifting the objective lensin the radial direction of the optical disc; a focus servo section forshifting the objective lens in a vertical direction with respect to arecording surface of the optical disc; and a moving section for movingthe optical pick-up in the radial direction of the optical disc, whereinthe optical disc reproduction apparatus further comprises a systemcontroller, the system controller comprising: a DVD signal levelacquiring and storing section for moving the objective lens upwards withthe CD laser diode turned off and the DVD laser diode turned on, andacquiring levels of the first, second, third, and fourth detectionsignals for DVD use from the photodetector for storage in the memory; aDVD ratio calculating section for calculating a ratio (A+D)/(B+C) forDVD use in a direction of the X-axis when the levels of the first,second, third, and fourth detection signals for DVD use stored in thememory are taken to be A, B, C, and D, respectively; a DVD lens shiftcorrection amount calculating section for calculating a lens shiftcorrection amount for the calculated ratio for DVD use based on acorrection table indicating a relationship of the calculated ratio forDVD use and the lens shift correction amount for DVD use when thecalculated ratio exceeds a predetermined value; a DVD lens shift sectionfor controlling the tracking servo section in accordance with the lensshift correction amount for DVD use that is calculated so as to shiftthe objective lens in the direction of the X-axis when the calculatedratio for DVD use exceeds the predetermined value; a CD signal levelacquiring and storing section for moving the objective lens upwards withthe DVD laser diode turned off and the CD laser diode turned on, andacquiring the levels of the first, second, third and fourth detectionsignals from the photodetector for storage in the memory; a CD ratiocalculating section for calculating a ratio (A+D)/(B+C) for CD use inthe direction of the X-axis when the levels of the first, second, third,and fourth detection signals for CD use stored in the memory are takento be A, B, C, and D, respectively; a CD lens shift correction amountcalculating section for calculating a lens shift correction amount forthe calculated ratio for CD use based on a correction table indicating arelationship of the calculated ratio for CD use and a lens shiftcorrection amount for CD use when the calculated ratio exceeds apredetermined value; and a CD lens shift section for controlling thetracking servo section in accordance with the lens shift correctionamount for CD use that is calculated so as to shift the objective lensin the direction of the X-axis when the calculated ratio for CD useexceeds the predetermined value, wherein the system controller executesprocessing provided by said sections included therein and reflectsresults in a succeeding series of processes required for achieving afocus-on state.
 6. The optical disc reproduction apparatus of claim 5,wherein the system controller further comprises: a DVD sum signal/focuserror signal acquiring section for acquiring a DVD sum signal (A+B+C+D)for the first, second, third, and fourth detection signals and a DVDfocus error signal (A+C)−(B+D) with the CD laser diode turned off andthe DVD laser diode turned on, after reflecting results of processing bythe DVD signal level acquiring and storing section, the DVD ratiocalculating section, the DVD lens shift correction amount calculatingsection, and the DVD lens shift section; a CD sum signal/focus errorsignal acquiring section for acquiring a CD sum signal for the first,second, third, and fourth detection signals and a CD focus error signalwith the DVD laser diode turned off and the CD laser diode turned on,after reflecting results of processing by the CD signal level acquiringand storing section, the CD ratio calculating section, the CD lens shiftcorrection amount calculating section, and the CD lens shift section; aDVD signal level adjusting section for individually adjusting the levelsof the signals A, B, C, and D so as to all become the same level withthe CD laser diode turned off and the DVD laser diode turned on when itis determined that an inserted disc is a DVD because a level of the DVDsum signal acquired by the DVD sum signal/focus error signal acquiringsection is larger than a level of the CD sum signal acquired by the CDsum signal/focus error signal acquiring section; a CD signal leveladjusting section for individually adjusting the levels of the signalsA, B, C, and D to all be the same level with the DVD laser diode turnedoff and the CD laser diode turned on when it is determined that theinserted disc is a CD because the level of the CD sum signal is greaterthan the level of the DVD sum signal; a DVD S-shaped signal balanceadjustment/focus pulling-in setting section for performing S-shapedsignal balance adjustment of the DVD focus error signal and settingfocus pulling-in in order to pull in a focus servo with the CD laserdiode turned off and the DVD laser diode turned on, after reflecting theresults of processing by the DVD signal level acquiring and storingsection, the DVD ratio calculating section, the DVD lens shiftcorrection amount calculating section, and the DVD lens shift section;and a CD S-shaped signal balance adjustment/focus pulling-in settingsection for performing S-shaped signal balance adjustment of the CDfocus error signal and setting focus pulling-in in order to pull in afocus servo with the DVD laser diode turned off and the CD laser diodeturned on, after reflecting the results of processing by the CD signallevel acquiring and storing section, the CD ratio calculating section,the CD lens shift correction amount calculating section, and the CD lensshift section.